The present invention relates to an ice melting system for thawing ice applied to a motor vehicle windshield such as a front windshield.
One proposed ice melting system for thawing ice applied to the front windshield of a motor vehicle in cold climate comprises an electric heater in the form of a thin film of silver oxide which is deposited on the front windshield by sputtering. When the electric heater is energized by electric energy supplied thereto, it is heated to increase the temperature of the glass of the front windshield, thereby thawing the ice.
The electric heater of the type described has a high electric resistance, and requires a large amount of electric energy to thaw the ice on the front windshield. The ice melting system needs to incorporate a large-capacity power supply which can supply a large amount of electric power ranging from 1.5 to 2 KW at a voltage from 60 to 70 V, for example.
There have been proposed ice melting apparatus in which an alternator for charging a battery on a motor vehicle is used as a power supply and the electric energy generated by the alternator is supplied to an electric heater to melt the ice on a windshield. Such proposed ice melting apparatus are disclosed in Japanese Patent Publication No. 61(1986)-33735 (corresponding to U.S. Pat. No. 4,084,126) and Japanese Laid-Open Patent Publication No. 63(1988)-69500 (corresponding to U.S. Pat. No. 4,780,619), for example.
The proposed arrangement disclosed in the former publication comprises a windshield heater circuit which is composed of a generator drivable by an engine on a motor vehicle, a windshield having an electric-resistance heater element, and a battery on the motor vehicle. The windshield heater circuit further includes a first switch for connecting the heater element to the generator, and a second switch that can be shifted between a first condition in which the output of the generator is connected to the battery and a voltage regulator for the generator and a second condition in which the voltage regulator is connected to the battery. In a thawing mode of operation, the first switch is closed to connect the heater element to the generator, and the second switch is shifted into the second condition, so that the voltage regulator is supplied with only the voltage from the battery.
In the thawing mode, as described above, only the battery voltage is applied to the voltage regulator, and the output voltage of the generator is controlled depending on the battery voltage. However, it is difficult to control the generator output voltage so as to be appropriate for melting the ice. It is desirable to obtain a certain voltage suitable for ice melting without resorting to feedback control. To meet this demand, the rotational speed of the engine which drives the generator, the capacity of the generator, and the load on the heater element have to be preset to strict values. Since these factors or parameters vary from device to device, it is highly difficult to obtain an amount of electric energy which is suitable for ice melting without a feedback control loop.
The latter publication shows a windshield heater circuit in which the AC output from a generator is boosted by a three-phase transformer to a higher voltage which is rectified into a DC voltage that is applied to a heater element mounted on a front windshield. Since the transformer is employed to boost the AC output from the generator, however, the disclosed windshield heater circuit is heavy and expensive. Use of the rectifier which rectifies the higher AC voltage into the DC voltage causes an electric power loss.